The present disclosure generally relates to welding power systems, of which welding power supplies is one type. More particularly, the disclosure relates to welding power systems used in situations utilizing purge gas, such as the welding of corrosion resistant metal containers, typically pipes and tubes.
During the welding of steels, oxidation occurs in the weld seam if oxygen reaches the weld area. This situation is even more acute with stainless steel, titanium, zirconium, molybdenum and other gas-reactive metals and alloys. The resulting oxidized surfaces are no longer corrosion-resistant, and further treatment is necessary.
To avoid costly and not necessarily satisfactorily effect subsequent treatment of the welds, the atmosphere around the weld is purged of oxygen. This reduces the amount of oxygen to levels that do not adversely affect the welds.
With purging, a gas is used to protect the weld seam until it has cooled to the point that oxidation no longer can occur. Usually, an inert gas, such as argon which is heavier than air, is used. Other purging gases such as nitrogen and nitrogen/hydrogen blends can also be used.
With pipes and tubes, the seam to be welded can be isolated using dams inserted into the pipes and tubes. The dams create a volume within the pipes or tubes within which the weld seam is located. The volume is determined by the diameters and lengths of the pipe or tube sections involved. For non-tubular shapes, the appropriate parameters for determining the volumes of the interior spaces are used.
The flow rate of the purge gas is important, if not critical. Too high of a flow rate results in unwanted turbulence of the air and an undesirable mixing of the oxygen into the purge gas. at which the welder should purge. Actually, the flow depends largely on the volume to be purged.
Typically, the purge gas flow is set to be just high enough to gently force the oxygen out of the volume to be purged and maintain a pressure inside the volume that is sufficiently higher than the pressure outside of the volume. This prevents excess turbulence in and re-entry of oxygen into the purged volume.
An important consideration is the purge time needed to create oxygen level conditions suitable for good welding. The purge time is a function of flow rate and the size of the volume to be purged. A general formula is: PT=(4×D×L)/PR, where PT is the purge time (typically minutes), D is the diameter of the pipe, L is the length of the pipe and PR is the purge rate (typically in cubic feet per hour).
Of course, the ultimate determinant of end of purging is the amount of oxygen within the volume. This is determined by one or more oxygen sensors, and can vary depending upon the type of metal involved. Welding conditions are usually reached when the oxygen sensor indicates a suitable value such as below 70 PPM for stainless steel or below 50 PPM for titanium. Semiconductors can require a level below 10 PPM. Purging can continue until the weld seam has cooled sufficiently so that oxidation can no longer occur.
The time to purge to reach ready-to-weld oxygen conditions can be relatively long for larger volumes. For a 6 inch diameter piper, an a typical volume, this can be 40 minutes or so. Thus, there can be much down time for a welder, or much wasted time checking the oxygen sensor.
The welding parameters are usually set by specification and enforced by quality control.
Presently, the purge time to reach ready-for-welding oxygen conditions are calculated manually by the welders using a handheld calculator or in their head. They can also be specified on a specification sheet.